1) Field of the Invention
The present invention relates to a multiple bit rate optical communication method, an optical network unit and an optical line terminal, and relates, for example, to a technique suitable for use in a system in which a passive optical network (PON) is adopted.
2) Description of the Related Art
At present, as a subscriber optical fiber network system intended for subscriber homes such as ordinary households, for example, a system connecting an optical line terminal provided in a regional office set up in a central office or the like and optical network units set up in a plurality of subscriber homes using optical fibers is known. Among others, a configuration in which one optical fiber performing input/output of optical data signals from a regional office is branched off in a plurality of destinations by a power splitter, which is a passive element, and an optical network unit of each subscriber home is connected to each of the branched optical fibers is called a passive optical network (PON).
The PON system has been commercially available as a system that can perform data transmission at high speed between an optical line terminal and a plurality of optical network units.
Then, a system configuration shown, for example, in FIG. 12 can be cited as a form of communication network using the PON system.
A PON system 100 shown in FIG. 12 is comprised of an optical line terminal (OLT) 101, N (N is an integer equal to or greater than 2) optical network units (ONU) 102-1 to 102-N (denoted simply as an ONU 102 if not to be distinguished) corresponding to N subscribers #1 to #N a power splitter 104, an optical fiber 103 connecting the OLT 101 and the power splitter 104, and optical fibers 105-1 to 105-N (denoted simply as an optical fiber 105 if not to be distinguished) connecting the power splitter 104 and each of the ONU 102-1 to 102-N.
In the PON system 100, the OLT 101 is a device equipped with required communication control functions such as converting an electric signal into an optical signal to transmit the optical signal to the ONU 102 side in a predetermined downstream frame format and converting data transmitted as an optical signal from the ONU 102 in a predetermined upstream frame format into an electric signal for information delivery and the like.
The optical fiber 103 connected to the OLT 101 is branched off by the power splitter 104 provided midway through a transmission path and each branched optical fiber 105 is drawn into an individual subscriber home to be connected to each ONU 102.
The ONU 102 is a device that performs communication control operations such as communication with the OLT 101 and conversion between optical signals and electric signals.
Here, upstream and downstream data transmission between the OLT 101 and power splitter 104 is performed in both directions by wavelength division multiplexing (WDM) using one optical fiber 103. Meanwhile, a direction from the OLT 101 to the ONU 102 is downstream and that from the ONU 102 to the OLT 101 is upstream.
A downstream frame from the OLT 101 to the ONU 102, for example, is transmitted using time division multiplexing (TDM) as an optical signal in a 1.49 μm band at a single bit rate A, and frame synchronization information and management information in the downstream frame are detected by the ONU 102 to extract, based on the detected information, data in time slots individually assigned in advance. An upstream frame from the ONU 102 to the OLT 101, on the other hand, is transmitted from the ONU 102 in a timing provided by the OLT 101. That is, an upstream frame from each ONU 102 is transmitted using time division multiple access (TDMA) as an optical signal in the 1.31 μm band in a timing so that collision of upstream frames can be avoided.
The power splitter 104 plays a role of dividing a downstream frame from one optical fiber 103 to a plurality of optical fibers 105 (power branching) and collecting (multiplexing) upstream frames from the plurality of optical fibers 105 into one optical fiber 103.
Meanwhile, the PON system 100 has generally been under study for use to provide low-speed services such as a phone service. In recent years, however, to provide high-speed communication services such as a video phone service and a TV conference service that are faster than the phone service is increasingly demanded, and to provide low bit rate data (low-speed services such as the phone service) and high bit rate data (high-speed services such as the video phone service and TV conference service) by one PON system 100 is called for.
Japanese Patent Application Laid-Open No. 8-8954 shown below, for example, discloses a method of increasing service capacities in a point/multipoint optical transmission system that realizes an increase in service capacities by using a multi-rate/burst circuit that generates a signal having a different rate for each time slot.
According to the above technique, all ONUs 102 are equipped with the multi-rate/burst circuit in advance to provide low bit rate data and high bit rate data by one PON system and extension not only to low-speed services, but also to high-speed services is enabled by assigning a different transmission speed (bit rate) to a portion of time slots assigned to one ONU 102.
However, the method described in Japanese Patent Application Laid-Open No. HEI 8-8954 is incompatible with existing systems, because existing systems such as a gigabit-capable passive optical network (G-PON) standardized by ITU (International Telecommunication Union) do not adopt a transmission method in which the bit rate is changed for each service in a physical layer.
Here, the above Japanese Patent Application Laid-Open No. 8-8954 neither considers nor suggests compatibility with existing systems described above.
Moreover, since the technology described in the above Japanese Patent Application Laid-Open No. 8-8954 requires preparations (installation) of the multi-rate/burst circuit in all ONUs 102 in advance, even subscribers who do not want high-speed services must upgrade the ONU (such as installing the multi-rate/burst circuit), posing a problem of more complicated device upgrade and a burden of costs thereof.